This is the website used when searching for tickets for the express buses departing from terminals in Seoul. It can be a bit of a pain to use as there doesn't seem to be an English option and in addition, the website doesn't seem to run properly on Safari.
Despite that, here's a quick guide to the Korean you'll see if you need to use the site.
The main bit that you'll need to use is the section on the left which says '승차권예약 / 예매' (=Ticket reservation/advanced purchase).
Here is the Korean that you will need:
출발지: Departure Point
도착지: Destination Point
출발일시: Departure Date
출발시간: Departure Time
등급: Grade (Economy/Luxury)
조회: Inquiry
The next page will take you to the reservation page as you can see in the second image. You can see the bus timetable showing the bus times. Here's the titles of the columns in the chart:
출발시간: Departure Time
도착지 : Destination Point
버스등급: Bus Grade
운수회사: Operating Company
소요시간: Travelling Time
일반요금: General Price (not including discounts)
잔여좌석: Remaining Seats
선택: Select